1. Field of the Invention
The present invention relates to an image processing apparatus for performing a screen process for image data.
2. Description of Related Art
When performing the screen process to represent character images in medium tone in a printer, if a regular screen pattern is used, dots are output every specified pixels (points dotted by the screen pattern are referred to screen dots), so that as shown in FIG. 11, a phenomenon called jaggy that notches are seen on the outline area of each character by the screen pattern occurs. When jaggy occurs, the outline area of each character is hardly discriminated, and the visual resolution of the character image is lowered, thus the character image is hardly read.
To prevent an occurrence of jaggy, a method for discriminating a characteristic (characteristic indicating the image kind such as characters and figures) of each pixel from information described in a page description language (hereinafter abbreviated to PDL) and performing an image processing according to the characteristic is disclosed (for example, refer to Patent Document 1). According to the method described in Patent Document 1, for example, for pixels in the image area of characters giving priority to the resolution, a TEXT signal indicating characters is generated, and at time of image processing, in the image area where the TEXT signal is set, a high-resolution screen such as a 400 lpi screen is used, and in the other areas, a low-resolution screen such as a 200 lpi screen is used, so that the screens having different resolution are used appropriately like this, thus the resolution of characters is avoided from deterioration.
On the other hand, as shown in FIG. 12(a), when the outline angle is close to the screen pattern angle, the output interval of screen dots in the outline area becomes longer, so that a problem arises that a long period of jaggy occurs very conspicuously. To solve the problem, it is effective to control the outline angle and screen pattern angle not to become close to each other and shorten the period of jaggy.
As such a method, an art for extracting the outline area of an image on the basis of the PDL, performing a screen process using a line screen pattern in non-outline area, performing a screen process such as error diffusion requiring no angle in the outline area, or performing the screen process by a line screen pattern of an angle different from the ordinary angle according to the outline angle in the outline area, thereby controlling the outline angle and screen angle not to become close to each other is disclosed (for example, refer to Patent Document 2).
Patent Document 1: Japanese Unexamined Laid-Open Patent Publication No. H9-28 Patent Document 2: Japanese Unexamined Laid-Open Patent Publication No. 2004-4 However, at the print and output stage of a printer, when two or more dots are output by ink or toner, the density may vary with the position relationship of dots depending on the printer. For example, between a case that two points are adjacent to each other and a case that two points are apart from each other, the whole density of ink or toner may be different by two dots. The reason is that it depends on a frequency response of the printer, and for output of one dot, the response is slow and ink or toner is not output so much, while for continuous dotting, the response is improved and ink or toner is output satisfactorily. However, such a characteristic varies in degree with the printer.
In the art described in Patent Document 2, when the screen process is switched in the outline area, a phenomenon may occur periodically that in the outline area, dots output by a different screen pattern locally make contact or separate.
For example, as shown in FIG. 12(a), when only the outline area is switched to the 0 degree line screen pattern for the 90 degree line screen pattern, it can be confirmed that a phenomenon that in the output area, dots by a different screen pattern locally make contact or separate occurs periodically.
In consideration of the aforementioned output characteristic of the printer, in the part where the screen dots make contact with, the density increases locally, and if the density increase, regardless of periodically or disordered, is scattered in the outline area, it is seen as jaggy, and the image quality may be deteriorated. The period in which the dots make contact with each other, when the screen angle and outline area angle in the non-outline area are close to each other, becomes longer and jaggy becomes conspicuous much the more.
Further, it may be considered to perform a conituous tone (contone) process for the outline area and add an outline, though a problem that the periodical contact of the outline with the screen dots is seen as jaggy cannot be solved. Further, such an art is limited to a case of application of the line screen pattern and cannot be applied to the dot screen. Even if the dot screen pattern angle used in the outline area is made just different from that used in the non-outline area, as shown FIG. 12(b), a phenomenon that in the outline area, dots by a different screen pattern locally make contact or separate occurs periodically and a problem similar to that of the line screen pattern arises.